This invention relates generally to die heads for producing fibers and nonwoven webs by extruding a melted thermoplastic material into a stream of air, and more particularly concerns a die head having a slot opening extending along the length of the die head instead of individual orifices spaced along the length of the die head.
Forming nonwoven webs by melt-blowing is well-known in the art and is described in various patents and publications, including Naval Research Laboratory Report No. 4364, "Manufacture of Super-fine Organic Fibers" by V. A. Wendt, E. L. Boon, and C. D. Fluharty; Naval Research Laboratory Report No. 5265, "An Improved Device for the Formation of Super-fine Thermoplastic Fibers" by K. D. Lawrence, R. T. Lukas, and J. A. Young; U.S. Pat. Nos. 3,849,241 to Buntin, et al.; 3,676,242 issued to Prentice; and 3,981,650 issued to Page.
The above references as well as current technology in the area of melt-blowing generally employ an extruder to force a hot melt of thermoplastic material through a row of fine orifices in a die head into converging high-velocity streams of heated gas, usually air, arranged on each side of the extrusion orifices. A conventional die head is disclosed in U.S. Pat. No. 3,825,380 to Harding et al.
As the hot melt exits the orifices, it encounters the high velocity heated gas stream, and the stream of thermoplastic material is attenuated by the gas and broken into discrete fibers which are then deposited on a moving collector surface, usually a foraminous belt, to form a web of the thermoplastic material.
In accordance with conventional practice, the hot melt is extruded through a number of fine individual orifices aligned along the length of the die head. In order to assure the fineness of the fibers, the orifices must be of small diameter. Because of the small diameter of the orifices, it is necessary to provide filter means between the extruder and the small diameter orifices to assure that agglomerated particles, foreign particles, slag, carbon deposits and the like in the polymer melt do not reach the small diameter orifices and plug them. The filters in the die head must be periodically renewed in order to assure that sufficient back pressure is not built up to inhibit the extrusion process. Even when filters are provided in the die head, agglomeration, slag, or carbon deposits may still exist downstream of the filters and prior to the hot melt being extruded through the orifices.
Consequently, the orifices, in spite of all precautions, are subject to becoming plugged. Once the orifices have become plugged, it is necessary to remove the die head from the production line and burn it out in an oven. Such a procedure results in lost production time, high maintenance expenditures, and the necessity for duplicate die heads so that one can be used while the other is being cleaned.
In addition, fabrication of the die head itself with the small diameter orifices represents a significant capital cost due to the expense involved in forming the fine orifices in the die head, by drilling, by electro discharge machining, or by laser drilling.